Blender 2.9, is there a way to select the collection in outliner when an object in that collection is selected?
No solution, just a workaround: I sometimes use the fact that the collection which holds the selected object has got a highlighted object symbol. If there's too many levels of hierarchy visible in the Outliner I press A to select all, Home and Numpad - so everything is collapsed. Then it's easier to find the collection with the highlighted symbol.
If you want to transform all objects in a collection at once without using Shift+G in the viewport or RMB+Click in the Outliner it would be best to parent all objects to an empty per collection.
You can also instance a collection or create a linked duplicate. In the Outliner, RMB+Click on the collection you want to duplicate, then choose either Duplicate Linked or Instance to Scene from the context menu. Test both to decide what works best for your needs.